This invention relates to new propylene-ethylene block copolymer compositions. More particularly, it relates to propylene-ethylene block copolymer compositons which are excellent not only in impact-resistance, but also in transparency, stress-whitening resistance, gloss and tensile elongation, and processes for the production thereof.
Crystalline polypropylenes produced by using a stereoregular olefin polymerization catalyst are excellent in rigidity and heat-resistance, but have a poor impact resistance, particularly that at low temperatures, which leads to limited use in various fields. As a method to provide improved impact resistance at low temperatures, there has been proposed a process to block copolymerize propylene with other xcex1-olefins such as ethylene. However, the resulting block copolymers are more improved in impact resistance at low temperatures than crystalline polypropylenes, but provide lowering in rigidity, hardness, heat resistance, transparency, whitening resistance, gloss, tensile elongation and the like, whereby the use thereof is restricted.
In order to solve the above-mentioned problems encountered in the block copolymers, a number of methods have been proposed, for example, a method which comprises a first step of homopolymerizing propylene in the presence of a catalyst for a streoregular polymerization, a second step of copolymerizing a mixed monomer of ethylene and propylene, and then stepwisely repeating the propylene homopolymerization and the ethylene-propylene copolymerization. In relation to the above-mentioned multi-stage polymerization method, Japanese Patent Kokai 54-152095 discloses using a titanium trichloride solid catalyst, and Japanese Patent Kokai 58-201816 discloses using an organoaluminum compound and an electron donating compound in combination with a titanium tetrachloride solid catalyst.
There have also been proposed a propylene block copolymer consisting of a crystalline polypropylene block and an ethylene-propylene random copolymer block, wherein the crystalline polypropylene block content is 55 to 95 percent by weight and the intrinsic viscosity ratio of the both blocks as well as the glass transition temperature of the ethylene-propylene random copolymer block are restricted; and a polypropylene block copolymer which consists of a polymer block mainly comprising propylene and an ethylene-propylene random copolymer block, wherein the intrinsic viscosity ratio of the both blocks and the intrinsic viscosity of the ethylene-propylene random copolymer block are restricted, and wherein the resulting block copolymer is melt kneaded.
An object of the invention is to provide a propylene-ethylene block copolymer compositon which is improved in transparency, stress-whitening resistance, gloss and tensile elongation and has a high impact resistance.
Another object of the invention is to provide a method for the production of the propylene-ethylene block copolymer composition.
We have extensively studied on the improvement in the impact resistance of known propylene-ethylene block copolymers and the factors which affect transparency, stress-whitening resistance, gloss and tensile elongation of the copolymers, and have found that those referred to as xe2x80x9cblock copolymerxe2x80x9d in the prior art are in the state of a micro-blend in which a polypropylene segment and an ethylene-propylene copolymer segment are not chemically bonded and also that the propylene-ethylene block copolymers are formulated into a copolymer composition containing a true block copolymer as defined in a polymer chemistry in which the polypropylene segment and the ethylene-propylene copolymer segment are chemically bonded, thereby providing the improvement in transparency, stress-whitening resistance, gloss and tensile elongation, as well as impact resistance.
According to the invention, there is provided a propylene-ethylene block copolymer composition (E) which comprises 0.01 to 10% by weight of an A-B type propylene-ethylene block copolymer (C) consisting essentially of a polypropylene segment (A) and an ethylene-propylene random copolymer segment (B), and 99.99 to 90% by weight of a propylene polymer (D), wherein the A-B type propylene-ethylene block copolymer (C) comprises 5 to 80% by weight of the ethylene-propylene random copolymer segment (B) having an ethylene content of 10 to 90% by weight and the propylene polymer (D) comprises 60 to 95% by weight of a homopolymer of propylene or a copolymer of propylene containing a copolymerizable monomer therewith (D1) and 40 to 5% by weight of an ethylene-propylene random copolymer (D2).
Further, the present invention provides a process of producing a propylene-ethylene block copolymer composition (E) which comprises the sequential steps of:
a) continuously supplying an olefin polymerization catalyst and a propylene monomer to the top area of a tubular type continuous polymerization apparatus to produce a polypropylene segment (A);
b) continuously supplying an ethylene monomer to the intermediate area of the tubular type continuous polymerization apparatus in which the ethylene monomer and the propylene monomer unreacted in the step (a) are copolymerized to produce an ethylene-propylene random copolymer segment (B) and sequentially the segment (B) is chemically bonded to the terminal of the polypropylene segment (A) by a covalent bond to produce an A-B type propylene-ethylene block copolymer (C);
c) transferring the A-B type propylene-ethylene block copolymer (C) containing the olefin polymerization catalyst produced in the step (b) to a single- or multi-stage polymerization reactor equipped with an agitator;
d) supplying a propylene monomer or a mixed monomer of a propylene monomer and a copolymerizable monomer therewith to the polymerization reactor, followed by copolymerizing in the presence of the olefin polymerization catalyst and the A-B type propylene-ethylene block copolymer (C) to produce a propylene polymer (D1); and
e) supplying a mixed monomer of ethylene and propylene to the polymerization reactor, followed by copolymerizing in the presence of the olefin polymerization catalyst, the A-B type propylene-ethylene block copolymer (C) and the propylene polymer (D1) to produce an ethylene-propylene random copolymer (D2), wherein each polymerization time in the steps a) and b) is controlled within the range of from 0.01 to 10 seconds.